1. Field of the Invention
This invention relates to an apparatus and method for measuring electromagnetic energy, and more specifically the invention relates to an apparatus and method for providing measurements of electrical energy having multiple wavelength components of electromagnetic energy, and which further provides measurements of electromagnetic energy that exhibit reduced levels of coma and astigmatic aberration effects, and which additionally provides an adjustment apparatus and method for aligning the field of view for measurements of electromagnetic energy.
2. Description of Related Art
Methods and apparatuses for measuring electromagnetic energy are well known in the art. It is often necessary to simultaneously measure intensities of multiple wavelength components of electromagnetic energy at a single spatial point. In addition, it is often useful to measure intensities of multiple wavelength components of electromagnetic energy at multiple spatial points along a line using multiple one-dimensional arrays of detectors to provide simultaneous readings. Furthermore, it is also sometimes necessary to use multiple two dimensional arrays of detectors to provide simultaneous measurement of intensities of multiple wavelength components of electromagnetic energy at multiple spatial points over an area.
A color line scan camera is an example of a measuring apparatus that includes multiple one dimensional arrays of detectors to provide simultaneous measurement of intensities of multiple wavelength components of electromagnetic energy at multiple spatial points along a line. The color line scan camera is useful for providing images of material as it is transported in regular fashion across a conveyor.
Most color line scan cameras employ a beam-splitter to divide the electromagnetic energy into its component parts and route it to the appropriate line scan detector.
Beam-splitters come in an array of types and sizes. A common beam-splitter known in the art is a cube or prism beam type splitter. These beam-splitters are often expensive and may be difficult to employ in infra-red applications. Another approach is to use a plurality of plate-type dichroic mirrors selected to filter and route the electromagnetic energy to the appropriate detector or sensor. Unfortunately, plate-type dichroic mirrors often introduce coma and astigmatism when placed in a convergent beam. This astigmatism and coma, of course, degrades image quality.
Pellicles have also been employed in the construction of beam splitters. In this arrangement, a set of pellicles are arranged to filter and route the electromagnetic energy to the appropriate detector. The pellicle is effective in reducing the amount of coma and astigmatism but is problematic in construction and application. Unfortunately, pellicles are often very delicate and often formed from nitrocellulose plastic having a thickness of less than 10 micrometers. The thickness and delicateness of the pellicles restrict the types of coatings that may be applied. For this reason, often coatings which are necessary to achieve a desired filtering or reflective result for the given electromagnetic energy are not compatible with the pellicle. Alternatively, coated glass pellicles may be used with coatings that are incompatible with the nitrocellulose plastic. However, coated glass pellicles are also difficult to form in a workable fashion in a beam-splitter because the coatings may distort the surface of the glass, resulting in variations of surface flatness which can introduce measurement anomalies.
Additionally, beam-splitters may also introduce chromatic aberrations which result in acuity degradations of the measuring apparatus. These chromatic aberrations distort the field of view and often result in alignment errors between colors or wavelength components.
Despite these various methods and apparatuses, there continues to be a need for a measurement method and apparatus that is able to provide measurements without significant astigmatism and coma distortion, and to provide adjustment for the alignment of the field of view, in a cost effective manner.